Loosely coupled Wireless Local Area Networks (WLAN) supported by WLAN servers, such as enterprise servers or the like, in addition to providing a user with such amenities as high-speed wireless Internet access to an expanding universe of services can provide an inexpensive alternative for telephony services as well as other real-time applications. A user can carry a wireless device such as a subscriber unit, which may also be referred to interchangeably from time to time herein as a communication unit, or a multi-mode communication unit with multi-use, such as dual-use, capability. Accordingly, the multi-mode communication unit can provide voice communication over an enterprise WLAN server when the multi-mode communication unit is in a WLAN and over a cellular wide area network (WAN or cellular network) such as GSM or 3G when the multi-mode communication unit is outside of the WLAN or leaving the WLAN. As noted, additional networks requiring additional modes of operation may also be present.
When the multi-mode communication unit is linked or otherwise connected through a wireless link or connection to a first network, such as during a call or other communication over the first network, and is moving within range of a second network such as, for example, from a WLAN to a cellular network, a handover of the call, and thus the link or connection, from the first network to the second network is desirable. Handovers are preferably conducted with minimal undesirable effects, service degradation, or service interruption. Since preparation for making a connection to a new network may take a relatively long time on the order of, say 10 s, for example to bring up software and hardware necessary to operate within and register with the new system, a user can experience unsatisfactory service or, worse, a dropped call or link. Conversely, maintaining registration and communication with multiple systems at all times can seriously reduce battery life for the multi-mode communication unit, and may create problems associated with the management of an excessive number of registrations for multi-mode communication units which may not be connected with a particular service. Thus maintaining registration with more than one network when it is not necessary is impractical and inefficient.
As will be appreciated, a main target market for multi-mode equipment is the large enterprise. Employees using multi-mode communication units can, for example, use the same phone in the enterprise and in the cellular system. Preferably, the multi-mode communication unit uses the Wireless IP LAN (WLAN) while the employee is inside the enterprise giving significant cost savings for the enterprise. In accordance with various exemplary embodiments, operation of a multi-mode communication unit, in switching between systems, should be transparent to a user. Thus, when a user leaves or enters an enterprise, the multi-mode communication unit should automatically switch modes from WLAN to cellular or vice versa. It should be noted that notwithstanding the need to switch systems or networks, inbound calls addressed to the multi-mode communication unit must reach multi-mode communication unit within a reasonable time regardless of its location.
Therefore, to address the above described problems and other problems, what is needed is a method and apparatus for reducing a call set up time for a call to a multi-mode communication unit associated with switching between modes, such as the time associated with setting up a new connection to a multi-mode communication unit during or after switching between wireless communications networks such as a WLAN and a cellular network.